1. Field of the Invention
The invention relates to a semiconductor device, and particularly to a semiconductor device provided with a cavity.
2. Description of the Background Art
A certain kind of semiconductor device has an element formed in a cavity. Particularly, when an element having a movable portion is arranged in a sealed cavity, characteristics of the element are improved in some cases. For example, according to Japanese Patent Laying-Open No. 2007-085747, a capacitance type acceleration sensor that is a semiconductor device has an acceleration sensing unit inside a cap (cavity) having airtightness. This cavity is defined by a substrate, a joint frame formed on the substrate and the cap joined to the joint frame. An interconnection held between oxide films extends through the joint frame.
In a process of manufacturing the acceleration sensor, the joint frame is formed on a surface on which the interconnection is present. The surface bearing the joint frame has irregularities formed at an edge portion of the interconnection and depending on the thickness of the interconnection. Therefore, the joint frame formed on this surface likewise has irregularities at its upper surface. When the irregularities at the upper surface of the joint frame are excessively large, it is difficult to join closely the joint frame and the cap together, which results in a problem that airtightness of the cavity cannot be ensured without difficulty. In particular, when a large shift or deviation occurs between the interconnection and the oxide film overlapped together, the irregularities at the upper surface of the joint frame become large, which makes the above problem more serious. For reducing the irregularities at the upper surface of the joint frame in the above conventional method, it is necessary to reduce the thickness of the interconnection. Thus, the above conventional method suffers from a problem that it is difficult to achieve both the ensuring of airtightness of the cavity and the reduction in electric resistance of the interconnection.